bpf: split eBPF out of NET

introduce two configs:
- hidden CONFIG_BPF to select eBPF interpreter that classic socket filters
  depend on
- visible CONFIG_BPF_SYSCALL (default off) that tracing and sockets can use

that solves several problems:
- tracing and others that wish to use eBPF don't need to depend on NET.
  They can use BPF_SYSCALL to allow loading from userspace or select BPF
  to use it directly from kernel in NET-less configs.
- in 3.18 programs cannot be attached to events yet, so don't force it on
- when the rest of eBPF infra is there in 3.19+, it's still useful to
  switch it off to minimize kernel size

bloat-o-meter on x64 shows:
add/remove: 0/60 grow/shrink: 0/2 up/down: 0/-15601 (-15601)

tested with many different config combinations. Hopefully didn't miss anything.

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

init/Kconfig

@@ -1341,6 +1341,10 @@ config SYSCTL_ARCH_UNALIGN_ALLOW
 config HAVE_PCSPKR_PLATFORM
 	bool
 
+# interpreter that classic socket filters depend on
+config BPF
+	bool
+
 menuconfig EXPERT
 	bool "Configure standard kernel features (expert users)"
 	# Unhide debug options, to make the on-by-default options visible
@@ -1521,6 +1525,16 @@ config EVENTFD
 
 	  If unsure, say Y.
 
+# syscall, maps, verifier
+config BPF_SYSCALL
+	bool "Enable bpf() system call" if EXPERT
+	select ANON_INODES
+	select BPF
+	default n
+	help
+	  Enable the bpf() system call that allows to manipulate eBPF
+	  programs and maps via file descriptors.
+
 config SHMEM
 	bool "Use full shmem filesystem" if EXPERT
 	default y